Here's a method similar to how I test my mixture after siphoning some out of the tank.

Unfortunately in my limited research I've found that stations in Dallas the Ethanol content of my 93 is 1-12% out of 5 stations I've tested. I have limited access to E85 so I store it in a 30 Gallon container in my garage. Since E85 can really be between 70-90% Ethanol (mine tested out at 89%) and gas can be vary I would suggest at least testing the stations you regular fill up at. I use a similar calculator as gdub already mentioned to get close and then do a final check of whats in the tank after driving around for 15-20 minutes to get the mixture mixed up real nice.